This invention relates to transfer gearing for transmitting power between first and second members rotating about parallel axes in automotive transmissions, for example in which power from the output shaft of a main gearbox must be transmitted to a pair of propeller shafts respectively extending fore and aft of the vehicle. These propeller shafts are usually interconnected by differential gearing, and extend generally parallel to the main variable ratio gearbox output shaft. Typically, power is transmitted between a gear on the main gearbox output shaft and a similarly sized differential gear, to give an approximate 1:1 gear ratio. There are other instances where power must be transmitted from a first shaft to a second, generally parallel shaft in an automotive transmission.
In heavy vehicles such as trucks, this power transmission is often via an intermediate gear supported on a layshaft of a transfer gearbox. The diameter and/or lateral offset of the intermediate gear may be selected to provide the required centre spacing between the parallel shafts. Where a large centre spacing is required, the intermediate gear diameter must be made correspondingly large. As the maximum transmissible torque depends mainly upon the load per unit tooth width, the intermediate gear and the co-operating gears must each be made relatively wide for transmission of high torque loads.
High torque loads and large centre spacings thus lead to a large and heavy gear train between the main and transfer gearboxes. Large diameter gears have a higher pitch line velocity and hence tend to be noisier in operation than smaller gears.
For lighter 4xc3x974 xe2x80x9csports utilityxe2x80x9d vehicles, such transfer boxes virtually all use chain drives for power transmission between the parallel shafts. The driveline must operate at high RPM and because of the necessary gear diameter/operating centre distances a conventional single power path geared drive as used in trucks, for example, would involve unacceptably large gears and pitch line velocities. At the speeds concerned, chain drive has proven quieter than even precision ground gears and allows a more compact casing. However chain drives suffer from overheating problems and excessive centrifugal loadings if used at speeds above about 6000 RPM. Their power transmission capacity is therefore limited.
Theoretically, an alternative design approach would be to provide a pair of intermediate gears offset to either side of the plane containing the parallel shaft axes, these intermediate gears each meshing simultaneously with corresponding gears on the parallel shafts (e.g. in the main and differential gearboxes respectively). Assuming that all four such gears are perfectly concentric, with perfect tooth pitches and profiles, supported on shafts perfectly spaced relative to one another, journalled in perfect, play-free bearings, the whole being made from perfectly inelastic materials, the transmitted torque will be shared equally between the two intermediate gears, to provide parallel torque transmission paths. The intermediate gears and the co-operating gears in the main and differential gearboxes could thus theoretically be made correspondingly smaller and lighter.
However, commercially manufactured gearboxes are not perfect. The kinematic forces acting on the intermediate gears coupled with the elasticity of the materials of the gear assemblies means that in reality one of the intermediate gears tends to be forced inwards towards the plane of the input/output shafts, whilst the other intermediate gear tends to be forced outwards away from that plane. The intermediate gear forced inwards experiences a higher torque than the intermediate gear forced outwards.
Furthermore, dimensional inaccuracies in the various gearbox components means that in reality one of the intermediate gears will, when torque is applied, assume flank-to-flank drive contact with each of the two adjacent gears, whilst at that instant the opposite intermediate gear has not established drive contact. Thus at that time only one torque transmission path is effective. As the torque load increases, provided that dimensional inaccuracies are within acceptable limits, gearbox components will deform under load until mutual drive contact is established between all adjacent gears. However, torque sharing between the two transmission paths will be unequal, with the degree of inequality corresponding to the size of the dimensional inaccuracies. The proportion of the torque transmitted through each path may vary throughout the rotation cycle of the gearbox assembly, as the dimensional inaccuracies of each gear may vary cyclically.
Studies by NASA on helicopter gearboxes (see paper by Timothy L; Krantz xe2x80x9cA Method to Analyze and Optimize the Load Sharing of Split-Path Transmissionsxe2x80x9d, published in Design Engineering, vol. 88, Power Transmission and Gearing Conference ASME 1996, at pages 227-242) have shown that satisfactory torque sharing between two parallel transmission paths can be achieved if tooth flank position errors are controlled to less than 0.0005 radian. Under these conditions, the inequality of torque transmission might not vary beyond, say, 60:40, leading to worthwhile savings in gearbox size and weight. Such dimensional accuracy is achievable, certainly in aerospace and similar specialist applications where high manufacturing costs are not prohibitive. However the requirement for high dimensional accuracy means that such torque sharing arrangements are impractical for mass produced automotive gearboxes, where low cost is an important factor.
U.S. Pat. No. 6,035,956 discloses an axle for low platform town buses in which hub reduction gear trains are connected, one on each side, between the axle differential and respective offset stub axles carrying the bus road wheels. Each transfer gear train comprises a pair of intermediate gears providing parallel power transmission paths. An input gear fixed to a respective output shaft of the differential meshes with both intermediate gears simultaneously and is vertically movable so as to share torque evenly between the power transmission paths.
U.S. Pat. No. 5,896,775 concerns high reduction final drive gearing for a powered scooter or wheelchair, in which an input shaft is connected to drive a pair of ground wheels through a pair of torque sharing pinion gears. The pinion gears engage a further gear wheel connected to drive the ground wheels. In one embodiment, the further gear wheel contains a differential arrangement.
It has now been realised that plural power path arrangements incorporating even torque sharing capability are of significant benefit to transfer gearing elsewhere in automotive transmissions, in particular between the main variable ratio gearbox and the axle (differential) drive, and also in other locations xe2x80x9cup streamxe2x80x9d of the axle differential.
In accordance with the invention there is provided an automotive transmission comprising a transfer gear train for transmitting torque between an input rotatable member and an output shaft rotating about substantially parallel axes, the transfer gear train comprising an input gear rotatable with the input member, an output gear rotatable with the output shaft, and a pair of intermediate gears each held simultaneously in mesh with the input gear and transmitting torque to the output gear to provide two power transmission paths, characterised in that the output shaft drives differential gearing arranged to distribute driving torque to a pair of ground engaging wheels. Preferably, one of the gears in the transfer train is made movable in response to the transmitted torque so as to even out power transmission between the two paths. However such torque sharing can also be achieved by other means, such as by controlling gear tooth flank position errors to within acceptably low limits.
The input gear is preferably made smaller than the output gear so that the transfer gear train provides a reduction ratio. The differential gearing may therefore have a lower reduction ratio, even substantially 1:1. This enables it to be made considerably smaller and lighter. Pitch line velocities in the transfer gearing and in the rest of the transmission driven by it are also reduced, giving quieter operation. The sizes of the gears in the transfer gear train and differential can be smaller than in a conventional single power path driveline, which much reduces gear weight (roughly proportional to diameter squared) and very much reduces gear moments of inertia (roughly proportional to diameter cubed).
In current driveline designs for passenger cars, the axle bevel gear has a large diameter, typically providing a reduction ratio of about 3:1xe2x80x94even larger for heavy vehicles. Any diminution in this reduction ratio increases gear loadings xe2x80x9cup streamxe2x80x9d, including in the main gearbox and transfer gearbox (if present). Current driveline proportions therefore represent a trade off between minimum gearbox size and maximum acceptable axle ratio. The plural power path transfer gear train of the present invention provides a high capacity, compact power transfer path that can handle the higher torque loads arising from the use of low ratio axle differentials, and may itself be used to provide a reduction ratio, thereby reducing torque loadings on the main gearbox.
A smaller axle differential gives a greater ground clearance, which is important in off-road vehicles. Incorporating speed reduction in the transfer gear train also enables the overall reduction ratio of the transmission to be maintained, whilst using a smaller reduction ratio at the differential gearbox. This is beneficial in high performance vehicles such as racing cars, with engines operating at high RPM and which therefore require a high overall driveline reduction ratio. For such applications, the transfer gear train of the present invention again gives lower pitch line velocities and smaller, lighter, quieter, lower inertia differential gearboxes and final drivelines.
The two power paths enable the transfer gear train to be made smaller and lighter than a conventional transfer gearbox of equivalent duty. To facilitate assembly of the various gears in proper meshing engagement and to provide flexibility in the available gear ratios and in the centre spacing between the input and output members, a further intermediate gear is preferably provided in each power transmission path.
The torque responsive movement referred to above may for example be of the input gear. In one possible arrangement, the input shaft and the rotational axes of the co-operating intermediate gears all lie substantially in a common plane.
Where the various gears are spur gears, the input shaft may be made free to move (e.g. pivot) very slightly away from this plane. Then, when there is flank to flank contact between the teeth of the input gear and only one of the intermediate gears, the torque applied to the input shaft and the reaction at the contacting tooth flanks will form a couple causing the input shaft to move out of the common plane. This movement continues until there is flank to flank contact at the other intermediate gear as well. Even torque sharing between the two power transmission paths is therefore achieved.
Where the various gears are single helical gears, the input gear may be free to pivot about an axis normal to the common plane. Out-of-balance forces acting in the direction of the input shaft axis and arising from uneven torque sharing between the two power paths will rotate the input gear about the pivot axis in a direction tending to reduce the out-of-balance forces, and hence evening up the torque sharing. Operation of such a mechanism is more fully explained in GB 1434928. To reduce or substantially eliminate thrust loads on the gear shafts, the gears may be mounted to their shafts by helical splined connections, the helix being of the same hand and having the same lead as the gear teeth. The splined connection may itself allow the input gear to pivot about an axis passing through the input gear.
Similar arrangements are possible in which even torque sharing or compensation is provided by movement of the output gear.
The torque compensating gear may also be free to move slightly both in a direction normal to the common plane and/or along the axis of the input rotatable member, as described for example in relation to the output gears 24, 34 in EP 0244263. Another arrangement permitting such translational movements of a torque compensating gear is described below.